Research on the course of alcohol and substance problems in treated adolescents is limited largely to 1-year outcomes, Further, little is known about mechanisms of change underlying reductions in substance use because existing studies preclude fine-grained temporal analysis of substance use patterns, The proposed project will use intensive, time-limited short term follow-up over 1-year, combined with longer-term follow-up through 2-years to identify factors influencing course of substance use and problems, with an emphasis on alcohol, cannabis and tobacco. We will prospectively study a total of 300 adolescents (age 14-18), each with a parent collateral informant: 200 adolescents will be recruited from addictions treatment and 100 adolescents will be recruited from the community. The community sample will serve as a reference group to determine the clinical significance of changes observed in clinical participants. All adolescents will be assessed at baseline (within I week of treatment entry for clinical subjects). 6-, 12-, 18-, and 24-months. Clinical adolescents also will complete monthly phone follow-ups during the first year of follow-up. Monthly follow-along data during the first follow-up year and longer-term follow-up through 2-years will be used to address three specific aims: (1) to develop and validate a model of the course of substance use and problems that integrates shorter-term processes of response, remission, and relapse with longer-term processes of recovery and recurrence, (2) to characterize the time course and factors associated with the onset of different course processes for alcohol, cannabis and nicotine, (3) to identify pathways and predictors of the course of substance problems in treated youth. Results will provide important new information that will enhance the timing and content of substance use interventions for adolescents.